1. Technical Field
The present invention relates generally to fuel control systems and, more particularly, to a method of compensating for boil-off corruption in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
A flexible fueled vehicle operated in cold ambient conditions experiences a condition referred to in the art as boil-off. When cold starting a vehicle that is fueled with alcohol, some of the injected fuel remains in a liquid state and accumulates in the engine oil. When the oil temperature reaches the boiling point of alcohol, the alcohol therein begins to vaporize and is introduced into the engine through the crank case ventilation system. These vapors can potentially cause a high level of rich fuel corruption until all of the alcohol in the oil is vaporized.
U.S. Pat. No. 5,520,162, entitled "Boil-off for a Flexible Fuel Compensation System", hereby expressly incorporated by a reference, provides a flexible fuel compensation system including a method of methanol-type boil-off compensation. The methanol boil-off compensation method includes an initialization routine, a run mode routine and a shut-down routine. The method monitors a plurality of flags to determine when the methanol boil-off compensation should be used.
Although this and other prior art methods are effective for their particular applications, there is room for improvement in the art. For instance, previously, there was no provision for allowing the percent alcohol content learning system of the motor vehicle to function without being corrupted by the temporary introduction of alcohol vapors into the engine. This corruption causes the fuel content multiplier of the percent alcohol content system to be frozen at an incorrect value. This causes potential drivability deficiencies after the boil-off condition ceases.
Also, if the fuel composition multiplier is determined before the boil-off condition arose, there is no way to compensate for the additional alcohol introduced into the engine during the boil-off period. This also poses the potential for rich drivability deficiencies. Therefore, it would be desirable to provide a temporary fuel composition multiplier for low and high air-flows in the engine for yielding additional fuel control during a boil-off condition and also saving the pre-boil-off fuel composition multiplier value for subsequent use.